The MAX6978 serial-interfaced LED driver provides eight open-drain, constant-current sinking LED driver outputs rated at 5.5V. The MAX6978 operates from a 3V to 5.5V supply. The MAX6978 supply and the LEDs" supply can power up in any order. The constant-current outputs are programmed together to up to 55mA using a single external resistor. The MAX6978 operates with a 25Mb, industry-standard, 4-wire serial interface.
The MAX6978 includes circuitry that automatically detects open-circuit LEDs. Fault status is loaded into the serial-interface shift register when LE goes high and is automatically shifted out on DOUT when the next data transmission is shifted in.
The MAX6978 includes a fail-safe feature for safety related applications, which blanks the display if the serial interface fails. If the serial interface becomes inactive for more than 1s, all driver output latches are automatically cleared. This turns off all LEDs connected to the outputs. The shift register data is not disturbed. The outputs remain off until the driver output latches are updated with data turning them on, so recovery is automatic if the transmission failure is temporary. The watchdog function requires no software change to the application driving the MAX6978.
The MAX6978 uses the industry-standard shift-register-plus-latch-type serial interface. The driver accepts data shifted into an 8-bit shift register using data input DIN and clock input CLK. Input data appears at the output DOUT eight clock cycles later to allow cascading of multiple MAX6978s. The latch-enable input LE loads the 8 bits of shift-register data into an 8-bit output latch to set which LEDs are on and which are off. The output-enable input active-low OE gates all eight outputs on and off, and is fast enough to be used as a PWM input for LED intensity control.
The MAX6978 is one of a family of 12 shift-register-plus-latch-type LED drivers. The family includes 8-port and 16-port types, with 5.5V- or 36V-rated LED outputs, with and without open-circuit LED detection and watchdog. All versions operate from a 3V to 5.5V supply, and are specified over the -40°C to +125°C temperature range.
